INFECTION it is the entry, development and or multiplication of an infectious agent in body of man or animal. CONTAMINATION it is the presence of infectious agents on the body surface, clothes or articles of a person or on inanimate objects in the environment including water and food. CHAIN OF INFECTION DEFINITIONS INFESTATION it means that parasites present on the body surface (e.g. louse infestation) or invading tissues (e.g. round worm infestation) POLLUTION it is the environmental contamination that causes instability, disorder, harm or discomfort to the ecosystem; it can be due to chemical substances or energy as noise, heat or light. INFECTIOUS DISEASES CONTAGIOUS DISEASES COMMUNICABLE DISEASES disease of man or disease transmitted from one disease resulting from infection or infestation animal due to infection. person to another by contact, e.g. scabies. can be transmitted directly or indirectly from man to man, animal to animal, animal to man or from environment to man or animal SUBCLINICAL INFECTION LATENT INFECTION OPPORTUNISTIC INFECTIONS infection without manifestation the infectious agent lies dormant within the host without clinical manifestations a non-pathogenic agent cause disease when host immunity (detected only by lab however under certain circumstances, it declines test). reactivates and causes disease (e.g. peumocystis carini, CMV (e.g. Herpes Zoster and TB). and Toxoplasma). PATTERNS OF INFECTIOUS DISEASES OCCURRENCE IN COMMUNITIES SPORADIC ENDEMIC Scattered cases of a disease with unknown common source of inf. Constant presence of a disease (all over the year) in certain area e.g. bilharziasis in Egypt. EPIDEMIC PANDEMIC: Sudden appearance of a disease in certain area and in a specific time Epidemic spreads from one country to another in short time OR OR a disease occurs in excess of normal expectation based on past experience in the community. Epidemic occurs in different countries at the same time (e.g. influenza). OUTBREAK EXOTIC DISEASE A localized epidemic affects a large group in the community as school, camp, hospital, e.g. food poisoning outbreak. A disease imported into a country e.g. imported malaria. ZOONOTIC DISEASES ENZOOTIC DISEASES EPIZOOTIC DISEASE Endemic diseases occur among Epidemic diseases occur animals, e.g bovine TB. among animals Diseases naturally transmitted between animals and man e.g. rabies, anthrax, bovine TB, brucella. NATURAL HISTORY OF DISEASE It is the progress of a disease in an individual over time in absence of intervention It starts from exposure to the causal agents till one of 4 possible outcomes occurs: 1. Exposure has no obvious effects. 2. Subclinical effect. 3. An illness (disease) followed by recovery. 4. Progression of illness long term problems, irreversible damage/ death.
CHAIN OF INFECTION (INFECTIOUS CYCLE) A chain of factors necessary for: Development & Maintenance of any infectious disease in the community. Agents causing infectious diseases. (viruses, bacteria, protozoa, parasites, fungi). (1) CAUSATIVE AGENT COMPONENTS OF INFECTIOUS CYCLE 1. Causative agent. 2. Reservoir (source of inf.) Types Portal of exit Period of infectivity 3. Mode of transmission 4. Host Portal of entry Incubation period. Susceptibility & resistance Clinical features, complications & diagnosis Agent factors that affect disease transmission: 1- Infectivity. 4- Antigenicity. 2- Pathogenicity. 5- Resistance outside the body. 3- Virulence. INFECTIVITY The ability of agent to produce infection (invade & multiply) in a susceptible host. 2ry attack rate It is measured by 2ry attack rate: The proportion of exposed susceptible persons who become infected. Number of 2rycases Number of susceptibles Examples: - High infectivity: measles, Polio - Low infectivity: leprosy x100 PATHOGENICITY The ability of the organism to produce specific clinical reaction after infection (the proportion of infected persons who develop clinical disease). Ratio of clinical to subclinical case = It is measured by: Clinical cases Subclinical cases VIRULENCE The ability of organisms to produce severe pathological reaction (the proportion of persons with clinical disease who become severely ill or die). It can be measured by case fatality rate. The ability of organism to produce specific immunity (antibodies or antitoxins). It can be measured by: The Re-infection rate: Case fatality ANTIGENICITY (IMMUNOGENICITY) RESISTANCE Total number of deaths from a disease rate x100 Total number of casesof that disease very rare: measles, mumps & chicken pox. common: syphilis, common cold & gonorrhea. The ability of the agent to survive in adverse environmental conditions. Environmental resistance Low (fragile) Moderate High Examples Gonococci & Influenza virus Salmonella Mycobacterium TB.
(2) RESERVOIR OF INFECTION I- HUMAN RESERVOIRS (CASES - CARRIERS) CASES Persons with symptomatic illness Typical case: a person with typical manifestations of the disease. Atypical case (very important source of infection): a person with atypical manifestations (difficult to be diagnosed), e.g. polio & meningitis. Subclinical cases (very important source of infection): infected person without clinical manifestation (detected only by lab. tests). It may be due to high immunity &/or low virulence of organism. The following diseases have no carriers: Influenza Whooping Cough Measles T.B. Herpes Zoster (1) According to relation to the case: Incubatory Carrier Convalescent Carrier Infective during Infective during last few days of lp recovery Typhoid & Cholera. Cholera, Typhoid, diphtheria CARRIERS They are apparently healthy (no symptoms) Infected persons (having the organisms in their bodies) excrete them in their discharges disseminate infection to others. Carriers are dangerous because: (1) No clinical manifestation : not known by others, so no prequations taken. (2) Their number is much larger than cases. (3) In some dis., Carriers are: - Dangerous as food handlers & school personnel - Infective for a long time, even for life as HIV, typhoid & HBV 1) Relation to the case 2) Period of carrier state 3) Foci of infection Contact Carrier Contact of a case get subclinical infection (no overt disease) & develop carrier state with excretion of disease agent Cholera, meningitis, Typhoid, diphtheria. Classification of carriers: 4) Discharges that carry organism outside body 5) Flow of organisms outside body: Continuous carrier & Intermittent carrier Healthy Carrier Occurs in endemic infectious diseases, when person get subclinical infection from polluted environment and develop carrier state hepatitis A & polio. (2) According to period of carrier state: Transient carrier Temporary carrier Chronic carrier Infective only for few days (last few days of I.P) Infective for few weeks up to few months Infective for years Cholera Hepatitis B Typhoid, Hepatitis B (3) According to foci of infection & discharge that carry organism
II- ANIMAL RESERVOIRS ROLE OF ANIMALS IN ZOONOSES (1) Animals are the only reservoirs of infection: no man-to-man inf. (e.g. plague, brucellosis). (2) Both animals & man are reservoirs of infection: Infection may be animal-to-man or man-to-man: e.g. Yellow fever: monkeys & man Salmonella food poisoning: man & (cattle, poultry, rats). (3) Animal is intermediate hosts & man is definitive host e.g. taenia saginata. (4) Animal is definitive host & man is affected by intermediate stage e.g. hydatid disease. EXAMPLES OF ZOONOSES 1. Cattle: e.g. anthrax, bovine TB, Q fever, salmonellosis, brucellosis 2. Poultry: e.g. avian flu, histoplasmosis. 3. Dogs: rabies, leptospirosis, hydatid disease. 4. Rodents: e.g. rabies, plague, weil s disease, relapsing fever 5. Fish: e.g. H. Hyterophyes, diphyllobothrium latum. 1. Plants: 2. Soil: agents live and multiply in soil, e.g. tetanus & anthrax. 3. Water pools: the primary reservoir of legionnaires bacillus III- Environmental reservoirs N.B. Reservoir may or may not be the source of infection e.g. soil is reservoir for clostridium botulinum but the source of botulism is caned food contain C. botulinum spores. (3) PERIOD OF INFECTIVITY (4) MODES OF TRANSMISSION Droplet infection Food borne infection Contact infection Arthropod borne infection Direct transmission Direct contact as in kissing Direct droplet: where host within the range of sprayed aerosol (max 1 mtr) e.g.: 1. common 3. TB cold 4. Measles 2. influenza 5. mumps 1- Droplet (respiratory) infection Indirect Droplet Air born infection Objects Milk Dust Droplet nuclei As towels, toys, utensils, brushes contaminated by resp. discharges & used in common Re-suspended particles from floors (sweeping, wind). e.g.: Fungal spores, TB Dried droplets (< 5 µ in size) coughed or sneezed into air remain suspended for long time. e.g.: TB If contaminated, it can cause respiratory infection through direct invasion of upper respiratory mucosa, e.g. diphteria, strept. Infection.
Inf. Agents in human excreta of case/carrier are transmitted to: - The patient himself (auto inf.), e.g. entrobiasis - A new host. Inf. Transmitted by: Hands & fingers: Hand-to-mouth inf. Ingestion of contaminated food, water, milk & drinks Viral 1- Poliomyelitis 2- Viral A hepatitis 3- Rota viruses & other virus causing diarrhea in children Inf. of undamaged surface Direct contact scabies, anthrax, STDs 2- Food-borne infection Food & drinks get contaminated through: 1. Handling of food by contaminated hands. 2. Flies & cockroaches. 3. using of fresh human fertilizers for vegetables. 4. Contaminated dust. Common vehicles of food borne diseases 1- Water-borne disease 2- Meat-borne disease 3- Milk-borne disease Water-borne disease Infectious Parasitic Bacterial Human excreta 1- Typhoid & paratyphoid 1- Ascaris eggs 2- Salmonellosis 2- Intestinal protozoa: 3- Shigellosis Entameba histolytica 4- Cholera Balantidium coli Giardia Lamblia N.B. Vertical transmission From mother to child. Transplacental after 3 ms of pregnancy. Insects of medical importance include: - Mosquitoes - Flies - Fleas - Ticks 3- Contact infection (through skin & mms) Inf. of injured surfaces Wound inf. Animal Bite tetanus rabies Dog excreta Echinococcus granulosus eggs Hydatid disease - HIV - syphilis - TORCH: Toxoplasma, rubella, CMV & herpes. Injection inf. Bl, drugs, vaccines HIV, HBV, HCV 4- Arthropod borne infection They transfer inf. agents from the source of inf. to another susceptible host. Transmission by a vector could be either 1- Mechanical transmission 2- Biologic transmission Mosquitoes House flies Sand flies Anopheles Culex Aedes Food-borne inf. Leishmaniasis Malaria Raft valley fever Encephalitis Filariasis Yellow fever Dengue fever Eye inf. Sand fly fever Fleas Lice Soft ticks Hard ticks Mites Epidemic typhus Tick-borne relapsing fever Q fever Lice-borne relapsing fever Spotted fever Tularaemia Trench fever Plague Murine typhus H. Diminuta Scrub typhus Rickettsial pox
The host is a person (or living organism) that can be infected by an inf. agent under normal conditions. (5) THE HOST 1- Portal of entry. 2- Incubation period. 3- Susceptibility & resistance. 1- Portal of entry The way though which agent enters susceptible host. It provides access to tissues in which agent can multiply (or its toxin can act). Usually, portal of entry to new host is similar to portal of exit from source host. e.g. Resp. T. as exit Resp. T. as entry 2- Incubation Period The period (time) From: Entry of inf. agent (or its toxin) into human body To: appearance of earliest clinical manifestations of dis. Length of IP depends on: 1- Portal of entry. 2- Rate of agent growth in host. 3- Dosage of agent. 4- Host resistance. I.P shortest longest Ex 1- Staph.Food poisoning 2- Salmonella food poisoning 3- Botulism 1- Rabies 2- AIDS 3- Leprosy IP 2-6 hrs 6-12 hrs 12-36 hrs 10 days - I yr 3-5 yrs Up to 10 yrs 1. To trace source of inf. e.g. in food poisoning. 2. To control contacts of cases by: - Surveillance for maximum l.p. (typhoid, diphtheria) - Isolation (cholera, pneumonic anthrax & pneumonic plague) A susceptible host is the final link in the chain of inf. General factors that susceptibility are: Malnutrition Alcoholism Disease or therapy which impairs immune response (AIDS, Cortisone, cytotoxic drugs) Epidemiological importance of I.P. (3) To set up: - International measure for quarantinable dis. cholera, yellow F. - Specific protection of exposed individual e.g. in exposure to small pox, person can be vaccinated in early I.P as immunity produced after 8 days while IP is 14 days. 3- Susceptibility & resistance Factors affect susceptibility & resistance to diseases: (1) Host factors: - Genetic factors - General factors: age, sex, education, occupation - Defense mechanisms: specific & nonspecific (2) Place; rural, urban, local or broad geographical differences. (3) Time ; seasonal, periodic, secular trend or epidemic (4) Environment; physical, biological or social. Defense mechanisms Nonspecific Defense Mechanisms Specific (Immune System) 1st line of defense 2nd line of defense 3rd line of defense Skin, mms & its secretions Phagocytic WBCs Antimicrobial ptns Inflammatory response Lymphocytes Antibodies